A fuel cell is an electrochemical cell that converts the energy stored in a source fuel into electric current. An electrolyzer can be regarded as a fuel cell operated in reverse, where an electrical current is provided to drive electrochemical reactions (e.g., the dissociation of water into hydrogen and oxygen). For both fuel cells and electrolyzers, chemical catalysts are often utilized to provide important gains in performance.
Platinum catalysts are important in fuel cells and a wide range of industrial catalysis applications. Typical solid oxide fuel cells (SOFCs) are operated at temperatures above 700° C., at which temperature perovskite oxides are sufficiently active to serve as the cathode and perform the oxygen reduction reaction (ORR). It is desirable to operate SOFCs at lower temperatures, though at lower temperatures perovskites do not have sufficient catalytic activity and produce large voltage losses, thus new catalysts are actively being sought. In proton exchange membrane (PEM) fuel cells, platinum is a popular catalyst, and alternatives are usually platinum alloys with high Pt content.
Aside from cost, a further problem of platinum catalysts is their susceptibility to “poisoning” by strongly adsorbed gases. Since hydrogen is most often produced from fossil fuels, the fuel stream on the anode side of fuel cells often contains small amounts of hydrocarbons, sulfurous species, and a large number of other chemicals. Contamination of the fuel stream on the ppm level may result in catalyst poisoning. The most problematic fuel contaminants in terms of fuel cell performance are carbon monoxide and dihydrogen sulfide. These molecules react strongly with platinum and are difficult to remove. Alloys of Pt—Ru are more able to resist poisoning by CO, but still contain a large percentage of Pt. For intermediate temperature fuel cells, Ni is often used as the anode catalyst. Nickel is poisoned even more severely by CO and H2S than Pt is.
Another issue with platinum catalysts is that operation over time causes agglomeration of disperse nanoparticle Pt, which drastically lowers the activity by decreasing the amount of available surface area for electrochemical reaction and results in a diminishing performance over time. Due to the high cost of Pt and its tendency to be poisoned and degrade, alternatives to Pt catalysts are actively being sought.
Representative examples of the state of the art include U.S. Pat. No. 7,422,994, where PtCuW and oxides, carbides and salts thereof are considered as fuel cell catalysts. Another example is given in US 2002/0004453, where suboxides are employed as fuel cell catalysts. A further example is given in U.S. Pat. No. 7,351,444, where nano-structured layers of PtVCoNi alloys are employed as catalysts.
However, it remains desirable to provide catalysts having a good combination of low cost, high performance and resistance to poisoning, and it would be an advance in the art to provide such catalysts.